One mechanism for monitoring the health of medical patients is blood gas analysis. This is a commonly performed procedure in the healthcare industry. A blood sample can be obtained from either a vein or an artery of a patient, depending on the information to be determined. The blood sample is then tested to determine information about the health of the patient. This information includes the content of the blood gas, for example, oxygen and carbon dioxide, and of ionized elements, for example, ionized calcium, sodium, and potassium. The status of the patient is determined by comparing the determined values from the sample to "normal" values.
A blood sample is obtained using a sampling syringe. The sampling syringe comprises an elongated tube open at one end with a hypodermic needed attached at the other end for communication with the tube. A plunger rod extends into the open end of the tube. A plunger cap attaches to the end of the plunger rod that inserts into the tube. The plunger cap facilitates drawing the sample of blood, as discussed below, and seals the sample from communication, for example, with the atmosphere, to prevent contamination and spilling of the sample from the syringe. Typically the plunger cap comprises an air impermeable body that encloses a self-sealing air permeable filter. The body forms a seal at a front end of the plunger. The front end includes an inlet into the filter which communicates with an exit port. A second seal closes the exit port for a purpose discussed below.
A syringe with venting capability is commonly used for drawing a blood sample from a patient. The venting syringe allows displaced air from the needle and the tube to vent to the atmosphere from the tube through the exit port. Closing the exit port with the second seal facilitates obtaining a fluid sample by aspiration, as discussed below. The sample is held in a sample chamber in the tube. The sample chamber is defined by the front face of the plunger cap and the end of the tube to which the needle is attached.
After a sample is drawn, the plunger cap then seals to prevent communication into and out of the tube. To do this, the filter includes a hydrophilic gelatin. When blood contacts the filter, the gelatin absorbs some of the blood fluids and solidifies to seal the plunger cap. This seal blocks fluid flow through the plunger cap. This protects the sample from contaminants in the atmosphere and reduces the transfusion of dissolved gases between the blood sample and atmosphere. The sealing action prevents blood from leaking out of the syringe through the vent. The seal also protects the sample from contamination by ambient air entering into the tube. Contamination of the sample or transfusion of the dissolved gases between the sample and the atmosphere results in incorrect analysis of the sample. This may lead to incorrect diagnosis and treatment.
The venting syringe is used to obtain fluid samples, such as blood, from a patient. In addition to venting and sealing, the sampling syringes have other needs that arise from how the syringe is used to obtain the sample. Particularly, sampling syringes are used to aspirate the patient for filling the sample chamber with blood, and the sampling syringes are used to pulsate self-fill with a sample. As discussed above, the sampling syringe must vent to allow displaced air to escape when the blood enters the sample chamber of the tube. The venting of displaced air occurs during or after collection of the blood sample. For example, during collection of an arterial blood sample, therapists typically preset the plunger rod in the tube for a desired sample size. The sample chamber in the tube is full of air. The needle is inserted into the target artery. Blood then begins to fill the sample chamber and displace the air. The air escapes from the tube through the exit port in the plunger cap.
Some fluid samples from patients are obtained by aspiration. This technique is particularly important for obtaining a blood sample from a patient with low blood pressure. During collection of a venous blood sample or an arterial blood sample from a patient with low blood pressure, the blood must be aspirated, or pulled, into the tube. The sampling syringe therefore must create a vacuum when the plunger assembly is pulled outwardly. For this to happen, the vent path in the plunger cap must close, or air will flow into the tube during withdrawal of the plunger rod. This would prevent the syringe from forming the vacuum. The vacuum is necessary to physically aspirate blood from the patient, through the needle, and into the collection chamber of the tube.
Aspiration to obtain a blood sample is performed by positioning the plunger cap fully within the tube against the tube wall where the hypodermic needle attaches. The needle is inserted into the patient and the plunger rod pulled outwardly. The second seal forms a fluid-tight seal that blocks communication through the plunger cap. The movement of the plunger rod creates a vacuum within the sample chamber. The vacuum aspirates, or pulls, the blood from the patient into the sample chamber.
Once the blood sample has been collected by aspiration, or by pre-set venting discussed above, a bubble of air typically lies between the blood sample and the port in the plunger cap. This bubble of air comes from the air in the needle and the syringe lower tip between the needle and the plunger cap. This bubble of air must be evacuated from the sample chamber with minimal contact to the blood sample. To leave or mix the air bubble with the blood would contaminate the sample.
The bubble of air is expelled in two ways. One way is to point the needle upwardly. This causes the air bubble to float through the blood sample up to the needle. The air can be expelled through the needle by advancing the plunger. This forces the air and some blood outwardly, preferably into an absorbent material for disposal. This method, however, has significant drawbacks. If an air bubble travels through the blood sample, the entire sample will be contaminated by diffusion of gases between the air bubble and the blood. Also, blood-borne pathogen presents a hazard from blood spray and from accidental needle sticks.
Venting syringes prevent these problems by expelling the air through the open end of the tube. The needle tip is first plugged with a standard needle stopper or cap. The plunger rod is then advanced inwardly against the air bubble and the collection chamber. The air bubble is thereby forced out through the exit port of the plunger cap without further contact with the blood. To do this, the second seal must open to allow the air to escape. The capability to vent through the plunger cap therefore must be reversible. First, the second seal must close to aspirate the blood sample from the patient. Second, the seal must open to discharge the air bubble which can contaminate the sample.
As the plunger rod moves inwardly into the sample chamber, the air escapes through the exit port. The plunger rod then contacts the meniscus of the blood sample. Upon contact with the blood fluids, the gelatin in the filter rapidly cures to seal the plunger cap.
For patients with sufficiently high blood pressure, therapists often allow the sampling syringe to self-fill under the pulsating pressure of the blood in the arteries. The plunger assembly accordingly must move outwardly in response to the blood filling the tube at normal blood pressure. In this practice, therapists use the syringe with the plunger assembly initially advanced completely inward. This positions the plunger cap against the inner end wall of the tube where the hypodermic needle attaches. The needle is then inserted into the target artery. Blood flows under arterial blood pressure into the syringe. This displaces the small amount of air trapped in the needle and the needle hub on the syringe. The air escapes through the exit port in the plunger cap and the blood contacts the plunger cap. The hydrophilic gelatin in the filter reacts with the blood, and seals the plunger cap from further communication. This accomplishes the purposes discussed above of the plunger cap and minimizes the time the blood is exposed to contaminating air. Accordingly, venting sample syringes must move the plunger assembly outwardly as the syringe self-fills, until the blood flow is stopped by removing the syringe or until the plunger rod stops moving. Typically sampling syringes that are capable of pulsating self-fill include a stop inside the syringe tube. The stop halts the movement of the plunger rod when the sample chamber is filled.
The prior art describes sampling syringes which have venting plunger caps. Typically, the plunger caps includes flexible frustoconical sealing elements at the longitudinal ends. The sealing elements project axially and outwardly into engagement with an inside surface of the syringe for forming a fluid-tight seal. One plunger cap has a longitudinal channel formed in the body which communicates with an air permeable filter held in a front portion of the cap. The channel includes a seat in an opening adjacent the filter. A ball is disposed in the channel. Upon holding the syringe at a predetermined angle, the ball falls against the seat to block air communication through the plunger cap.
Another plunger cap includes a rearwardly-facing annular surface that partially closes a cavity in the cap. The plunger rod connects to the plunger cap by inserting a tip into the cavity. The tip attaches to a neck that extends from an end of the plunger rod. The thickness of the annular surface is less than the length of the neck, thereby permitting axial movement of the plunger cap with respect to the tip. An air-tight seal is defined by bringing a forwardly-facing surface of the tip into contact with the rearwardly-facing annular surface.
Another plunger cap includes passageways that extend through the cap for communicating air from the sample chamber out of the syringe. The passageways each include a hydrophilic thread which swells in contact with blood fluids for blocking communication through the passageways. This plunger cap provides venting but is unable to aspirate a sample.
The plunger caps for the sampling syringes described above are manufactured by one of two methods. One method molds the body of the plunger cap the filter in place. The other method molds the body and the filter separately. The two components are then assembled by inserting the filter into the body. Each method has benefits and drawbacks. When molding in place, the high temperature and the high pressure of the injected material for the body compresses the filter together. The filter material softens and compresses. This reduces the size of the pores through which the air communicates through the filter. The effectiveness of the plunger cap for venting air and for aspirating samples is lessened. On the other hand, separate manufacture and assembly requires additional labor. The plunger cap using the ball valve is believed to be manufactured in place, with the ball being inserted. Further, defective assembly of the plunger cap may prevent the plunger cap from forming the seal sufficient to aspirate a sample. The defective syringe causes additional discomfort to a patient who must have a second needle insertion to obtain the one sample, after the defective syringe is disposed of.
It is thus seen that a need remains for a plunger cap that vents displaced air from a sample chamber, that seals the vent for aspiration of a sample, and that pulsate self-fills a sample from a patient, with a method of manufacture that offers the benefits of mold-in-place manufacturing without the drawbacks that limit the effectiveness of the venting through the plunger cap, in a more effective and efficient manner. It is to the provision of such that the present invention is primarily directed.